Modern communications systems are ever more increasing in bandwidth, causing greater needs for broadband antennas. The simple ½ wave wire dipole antenna, which can have 2.0 to 1 VSWR bandwidth of only 4.5 percent, is often not adequate. Broadband dipoles are an alternative to the wire dipole. These preferably utilize cone radiating elements, rather than thin wires. A biconical dipole, having for example, a conical flare angle of ½π radians has essentially a high pass filter response, from a lower cut off frequency. Such an antenna provides great bandwidth, and a response of 10 or more octaves is achieved.
Wire dipoles can be easily constructed by various techniques, including modifications of coax cable. In one modification, shield braid is inverted over the coax cable outer jacket, to form the lower section of a sleeve dipole. The exposed center conductor then forms the upper half element of the dipole.
In current, everyday communications devices, many different types of conical antennas, such as biconical dipoles, conical monopoles and discone antennas are used in a variety of different ways. These antennas, however, are sometimes expensive or difficult to manufacture. A simpler method of realizing the bandwidth of conical antennas is needed, one that can utilize existing hardware, such as common flanged chassis type coaxial connectors.
Conical antennas, which include a single inverted cone over a ground plane, and biconical antennas, which include a pair of cones oriented with their apexes pointing toward each other are used as broadband antennas for various applications, for example, spectrum surveillance. A biconical antenna includes a top inverted cone, a bottom cone and a feed structure, as disclosed in U.S. Pat. No. 2,175,252 to Carter entitled “Short Wave Antenna”. An electronic coupler provides a connection to a feeding circuit that provides an electrical signal that feeds the antenna. The antenna is symmetric about the cone axis and each of the cones is a full cone, spanning 360°. Referring to FIG. 2, the antenna pattern beamwidth of a conventional biconical antenna is diagrammatically illustrated. As can be seen in the diagram, the beamwidth decreases as frequency increases. This may be undesirable for various applications.
Similarly, a single cone antenna includes a single antenna cone that also spans 360° and is symmetric about the cone axis. A single antenna cone is connected to an electronic coupler that provides a connection to a feeding circuit that provides an electrical signal to feed the antenna. The single cone antenna is located over a ground plane.
An example of a discone antenna is disclosed in U.S. Pat. No. 2,368,663 to Kandoian. The discone antenna includes a conical antenna element and a disc antenna element positioned adjacent the apex of the cone. The transmission feed extends through the interior of the cone and is connected to the disc and cone adjacent the apex thereof. Also, U.S. Pat. No. 4,851,859 to Rappaport discloses a discone antenna having a conducting cone with an apex and a conducting disc with a disc feed conductor extending from its center. The conducting disc is mounted at the apex of the cone in spaced relation therewith such that the disc feed conductor extends down into the cone through the cone's apex. A coaxial connector is mounted within the cone at the apex of the cone.
Conventional discone antennas may have broad VSWR bandwidth but they suffer from narrow pattern bandwidth because the pattern droops, i.e. radiates downwards or away from the target, as the frequency increases, as illustrated in FIGS. 1A-C. Furthermore, the attachment of the antenna feed is complicated due to the routing through the cone. Accordingly, there is a need for broadband antennas that do not suffer from these drawbacks.